Dimethyl terephthalate composition and process for producing the same

ABSTRACT

A dimethyl terephthalate composition includes 0.001 to 200 ppm of methyl 4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethyl hydroxyterephthalate contained in dimethyl terephthalate, and exhibits improved properties as a material for producing polyester.

TECHNICAL FIELD

The present invention relates to dimethyl terephthalate usable as astarting material for producing a polyester. More particularly, thepresent invention relates to a dimethyl terephthalate composition havingimproved properties and usable as a starting material for the productionof a polyester, and a method for producing the same.

BACKGROUND ART

The dimethyl terephthalate (which will be referred to as DMThereinafter) is a main starting material for production of polyethyleneterephthalate (which will be referred to PET hereinafter), which is apolycondensate product of DMT with ethylene glycol (which will bereferred to as EG hereinafter). The Witten-Hercules method can be citedas a typical method for producing DMT.

The method of the present invention for producing the DMT comprisesoxidizing p-xylene (which will be referred to PX hereinafter) and methylp-toluylate with air, esterifing the resulting oxidized reaction mixturewith methanol (which will be referred to MeOH hereinafter) underhigh-temperature high-pressure conditions, and collecting and refiningthe DMT from the esterifying reaction mixture.

However, there are problems that DMT produced from PX (which will bereferred to PX-DMT hereinafter) according to the above method causes ahydrolytic reaction to occur when the PX-DMT is brought into contactwith steam or the like, and acid components are formed as by-products,and, thus, the acid value of the resultant PX-DMT increases; and thatthe reaction product mixture contains a large amount of dimethylhydroxyterephthalate (which will be referred to as HDT hereinafter)produced as a by-product by the oxidizing reaction, and thus the productexhibits a high intensity of fluorescence due to the presence of HDT,and the color of the product is bad.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve the problems of the aboveprior arts and to provide a DMT composition having a controlled acidvalue, at a low level, and a good color.

BEST MODE FOR CARRYING OUT THE INVENTION

In the process of the present invention, firstly, methyl4-(1,3-dioxolan-2-yl)benzoate (which will be referred to 4-DOMBhereinafter) must be prepared. The process of the production of 4-DOMBinclude (1) a method for obtaining the 4-DOMB by synthesis and (2) amethod for utilizing a by-product from a process for recovering DMT froma polyalkylene terephthalate by using EG and MeOH. The object of thepresent invention can be achieved by using DMT including a small amountof the 4-DOMB, even derived from either one of the methods.

As the above synthetic method (1), 4-DOMB can be readily produced bysubjecting equimolar amounts of 4-carbomethoxybenzaldehyde and EG to athermal reaction procedure under ambient atmospheric pressure in thepresence of a known general-purpose acidic catalyst and at a temperatureof 80 to 150° C. In this method, it is important to remove watergenerated as a by-product by a dehydrating reaction. The removal of theby-product water can be effected by distilling off or by use ofmolecular sieves with high efficiency.

On the other hand, the method (2) for utilizing the by-product from theprocess for recovering the DMT is an extremely favorable method even inindustrial aspects because the by-product can be directly utilized andthe DMT composition containing the resulting 4-DOMB needs no subsequentrefining operation.

In the above method (2), initially, the polyalkylene terephthalate issubjected to a depolymerizing reaction in the presence of a knowndepolymerization catalyst in EG. The polyalkylene terephthalate usedherein includes polyethylene terephthalate, polytrimethyleneterephthalate and polybutylene terephthalate. If necessary, the mixtureobtained by the depolymerizing reaction is subjected to the proceduresthat an excessive amount of EG used in the depolymerizing reaction iswithdrawn from the mixture, and the withdrawn EG is introduced, togetherwith MeOH and a transesterifying reaction catalyst, into a reactor andsubjected to a transesterifying reaction to produce crude DMT and analkylene glycol. The resulting reaction mixture is subjected to coolingtreatment and then to a centrifugal separation treatment to separate acake of the crude DMT from the mixture solution.

A DMT composition containing the 4-DOMB in an amount within the range of0.001 to 200 ppm can be obtained by, (A) carrying out operations to addMeOH, in a weight of 1 to 5 times based on the resulting cake, to thecake under conditions of 30 to 60° C. and then separate the resultingmixture again into the cake of the crude DMT and a mixture solution withthe centrifugal separator, or (B) adjusting, for example thedistillation column top temperature to 160 to 210° C. and the pressureto 2.7 to 13.3 kPa and performing control so that the reflux ratio iswithin the range of conditions of 0.1 to 2 when the resulting cake isdistilled and purified. There is no problem at all if the methods (A)and (B) are used in combination.

As to the HDT, the HDT concentration must be minimized because the HDTconcentration in the DMT directly contributes to increasing thefluorescence intensity of the DMT. However, there are problems that,generally, the PX-DMT is produced by an oxidation reaction of PX andthus the HDT is produced as a by-product, and the DMT produced as afinal product contains 1 ppm or more of HDT.

However, when the method (2) utilizing the by-product of the process forrecovering the DMT, is employed a DMT composition containing the HDT ina content of within the range of 0 to 1 ppm can be obtained.

As mentioned above, by utilizing a DMT composition containing the 4-DOMBwithin the range of 0.001 to 200 ppm and the HDT within the range of 0to 1 ppm the increase in the acid value of the composition can beprevented, and a dimethyl terephthalate composition having a lowfluorescence intensity, good color, and improved properties as thestarting material for a polyester, can be obtained.

In the present invention, in the case of the above methods (A) and/or(B) in which the process for recovering the polyester is utilized, the4-DOMB can be added, without problem, by either one of a batch methodand a continuous method. Also, when the above-mentioned method (2) inwhich the by-product from the process for recovering the DMT is utilizedand the process is carried out under the following conditions, the DMTcomposition containing the 4-DOMB and HDT within the scope of thepresent invention can relatively easily be obtained.

Namely, the depolymerizing reaction of the polyester with EG may usuallybe carried out a temperature of 110 to 230° C. and a pressure (gaugepressure) of about 0.0 to 0.2 MPa. The depolymerizing reaction with EGis sufficiently carried out when the conditions are within theabove-mentioned range. When the depolymerizing reaction temperature isbelow 110° C., the necessary depolymerizing time is extremely prolongedand the depolymerization is not efficient. On the other hand, when thedepolymerizing reaction temperature exceeds 230° C., a reactor resistantto a high pressure must be employed and the depolymerization isunfavorable in consideration of the operational and the safety aspects.

When the transesterifying reaction with MeOH is conducted, the reactiontemperature is preferably within the range of 50 to 150° C., and thereaction pressure (gauge pressure) is preferably within the range of 0.0to 0.59 MPa. When the reaction temperature and reaction pressure arewithin the above-mentioned ranges, the transesterifying reaction can besufficiently carried out. The transesterifying reaction time ispreferably 30 minutes to 4 hours.

As either one of the depolymerizing reaction catalyst and thetransesterifying reaction catalyst, conventional catalysts can be used;however, at least one type of metal salt compounds selected from thegroup consisting of carbonates, hydrogencarbonates and carboxylates ofan alkali metal and an alkaline earth metal are preferably used, due tothe high catalytic activity thereof. Furthermore, sodium carbonate isespecially preferably used for both of the reaction catalysts.

In the mixture obtained by the transesterifying reaction, crude DMT,MeOH, EG and by-products produced during the depolymerizing reactionwith EG and transesterifying reaction are present and the mixturefurther contains 4-DOMB, HDT, and others, for example, dioxane, dimethylether, water, etc.

In the case where the above-mentioned mixture is directly used to carryout recrystallizing operation, or crystals of DMT are not completelydissolved, the mixture is heated to perform a dissolving operation witha mixture solution. In this case, the mixture kept at thetransesterifying reaction temperature is directly cooled to atemperature of 10 to 50° C. or, after the mixture is heated once to atemperature in the range of 60 to 150° C. in response to the compositionof the mixture, it is then cooled to 10 to 50° C. If necessary, a latentheat of the solvent is preferably utilized to cool the mixture.

The DMT concentration in the mixture solution is preferably kept in therange of 10 to 40% by mass. When the DMT concentration is less than 10%by mass, the necessary amount of the solvent to be used may increase andan economical disadvantage may occur.

The recrystallization-treated mixture is subjected to a solid-liquidseparation treatment using a centrifugal separator or the like, and thenthe resulting cake is washed with MeOH.

The amount of the MeOH mixed to the cake and the washing temperature arepreferably in the range of 1 to 5 times the amount of the crude DMT cakeand within the range of 30 to 60° C. to enhance the washing effects andhandleability of the solid-liquid slurry and economical efficiency. The4-DOMB contained in the crude DMT cake is incompletely removed by thewashing step. The number of the washing operations applied to the cakeis usually about one to three times but there is no problem when thenumber of the of washing operations is further increased.

After completing the washing with the MeOH and solid-liquid separation,the resultant crude DMT cake is heated and melted to thereby furtherremove the MeOH. The resultant DMT cake is finally distillation-refinedunder reduced pressure, and as a distilled fraction, a refined DMT iscollected.

The distillation-refining procedure is performed under conditions of areduced pressure of 2.7 to 13.3 kPa, a column top temperature of 160 to210° C. and a reflux ratio of 0.1 to 2.0.

The vapor pressure of the 4-DOMB is somewhat lower than that of the DMTand a trace amount thereof may be contained in the resultant DMTcomposition as long as the vapor pressure is within the above-mentionedrange. Excessively severe conditions in distillation-refining procedureare unfavorable because the detection of the 4-DOMB in the DMTcomposition becomes difficult. Also, an excessive intensification of theconditions should be avoided, because the distilling of the acidcomponents causes an increase in acid value of the DMT composition.

Therefore, in order to cause the 4-DOMB to be contained in an adequateamount in the recovered DMT, it is particularly preferable from aspectsof quality control that a distillation column having 5 to 20 platesexpressed in terms of theoretical plates is used, and the operationconditions of the distillation are kept at a column top temperature of180 to 195° C., in a reflux ratio of 0.3 to 1.0, and under a reducedpressure of 5.3 to 9.3 kPa.

On the other hand, as the amount of the HDT produced as a by-product inthe depolymerizing reaction stage is very small, the concentration ofHDT can be adjusted to a desired level by applying the above refiningprocedure.

In the present invention, the following system may be constructed toutilize the by-product produced in the above mentioned process forrecovering the DMT.

Namely, when a recycling system, for the polyester, comprising producingmaterials for the production of polyester having a quality identical tothat in trade by utilizing the process in which products available intrade and comprising as a principal component, a polyester, and wastesof products comprising a polyester as a principal component, andgenerated in the process for producing the polyester products arerecovered from the trade; the recovered products and wastes aredepolymerized with EG; and the depolymerization product istransesterified with MeOH to separate and refine the resultant crude DMTand EG; producing polyester products from the resultant materials forpolyester; and supplying and circulating the polyester products in thetrade, is constructed, it becomes possible to produce polyester productswith a high degree of purity from polyester wastes which are now finallytreated by incineration or landfilling disposal, and resources caneffectively be utilized. The system will be illustrated in detail below.

A pretreating step for polyester wastes for example, polyester bottles,comprising, as a principal component, a polyester will be explainedbelow. The wastes are pulverized preferably to a size of 2 to 30 mmsquare, more preferably 5 to 20 mm square, in the pretreating step fromaspects of reactivity of the wastes in the reaction step ortransportability of the wastes after the pretreating step. Thereactivity of the wastes is improved by the pretreatment. When thewastes include the bottles having thickened portions which are formed toincrease the mechanical strength and dimensional stability of thebottles by a crystallizing treatment or whitening treatment, theabove-mentioned effects significantly appear at the thickened portions.

In the pulverizing step, the pulverizing capacity is preferablyincreased by using a two step-type pulverizing apparatus. That is, thewastes are roughly pulverized to a size of 30 to 150 mm square with acrusher for a first step and then pulverized to a size of 2 to 30 mmsquare with a crusher for the second step. When the wastes are directlypulverized to a size of 2 to 30 mm square with the crusher for the firststep, a load applied to the crusher is too high and the pulverizingefficiency becomes low. When a magnetic separator for removing metalcomponents is installed upstream to and/or downstream from the crusherfor the first step, there are effects on a reduction in load on a cutterof the crusher for the second step. There are many cases where thepulverized material is contaminated with plastics different from thepolyester such as polyethylene, polypropylene, polystyrene or polyvinylchloride, contained as impurities in the wastes, which are used asmaterial of caps, labels or the like for the polyester bottles.

In the system of the present invention, when the polyester wastes arecontaminated with the above-mentioned different plastics, the reactionconditions can be established so that the purity of the recoveredproducts is not decreased by a decomposition of the different plasticsin the later reaction step. However, as the different plastics have apossibility of exerting unfavorable influences upon handling such assticking to a reactor or causing clogging of filters, it is importantthat the different plastics are removed as much as possible in thepretreating step to suppress the contamination in the reaction step asfar as possible to make the reaction proceed smoothly. Various steps forcompletely removing the different plastics as in the case of carryingout the material recycling, however, are not necessary, and only thenecessity minimum steps are required.

It is preferable that the labels are initially removed from thepulverized wastes by air blow separation for removing a thin plasticfilm (polyethylene, polypropylene, polyvinyl chloride or the like) whichis different from the polyester and used in the labels or the like. Whenthe air flow rate is too high in this process, the polyester which is auseful component is removed in company with the labels, and thus, it isnecessary to suitably adjust the air flow rate. Most of the labelscomprising, as a principal component, polypropylene, polystyrene orpolyvinyl chloride, can be removed by air blow separation.

The pulverized wastes is preferably treated with a decanter in order toremove the caps, which cannot be removed by the air blow separation, toremove the different plastics, for example, polypropylene andpolyethylene, having a lower specific gravity than that of water by acentrifugal separation method. Since the decanter is used also asequipment for washing off impurities (soy sauce, soft drinks or thelike) which are derived from foods or the like and remain in the wastes,with water, there is no problem at all if the contents remain in, forexample, the bottles. The washing water separated by the centrifugalseparation is recycled again to the decanter and a portion of therecycled washing water is delivered and subjected to wastewatertreatment.

The pulverized wastes which are delivered from the decanter and washedwith water (which wastes will be referred to recovered flakeshereinafter) are transported to a reactor, for the reaction step, by apneumatic transportation means. When the size of the pulverized wasteparticles is adjusted to a relatively large size such as 30 to 150 mmsquare in the above-mentioned pulverizing step, problems such asdeterioration of transportation efficiency or choking of rotary valvesof the pneumatic transportation means occur. To solve the problems,preferably the wastes are pulverized to a size of about 2 to 30 mmsquare as in the present system. Moisture remains in the recoveredflakes before the pneumatic transportation; however, from aspects of thereactivity of the wastes in the reaction step, the moisture content ispreferably reduced to 0.2% or less based on the mass of the recoveredflakes by drying the wastes during the pneumatic transportation or usinga dryer or the like.

The pretreatment of the wastes comprising, as principal components,fibers, films or the like will be explained below. As the material, forexample, nylon, polyethylene, polypropylene or cotton, which aredifferent from the polyester cannot serve as an effective component, thewastes contaminated with a large amount of those material are preferablyremoved at a stage of acceptance. That is, the wastes are analyzed witha discriminating device such as a near-infrared ray spectrometer, andwhen the wastes exhibit an absorption pattern different from that of thepolyester, the wastes are removed in this stage, without transportingthe waste to the next step, to enhance the efficiency of the wastetreatment.

It is preferable that the wastes passing through the inspection with thenear-infrared ray analysis are granulated and solidified from aspects ofhandleability in the reaction step after the pretreating step orpneumatic transportation after the pretreating step. However, the directcharging of yarn wastes, film wastes or the like having a continuousstructure into a granulater is extremely difficult, and thus, the wastesmust be initially pulverized into an adequate size. The size of thepulverized waste particles is preferably 2 to 50 mm square. When thesize of the pulverized waste particles is too large, in the nextgranulating step, unfavorable results, namely an insufficientsolidification, occur. As a mode for carrying out the pulverization,preferably the pulverizing apparatus is of a two-stage type, to increasethe pulverization efficiency. Namely, the polyester wastes are firstcrushed into a size of 30 to 150 mm square with a first crusher and thenpulverized to a size of 2 to 50 mm square with a second crusher. Whenthe wastes are directly pulverized to a size of 2 to 50 mm square withthe first crusher, the load applied to the first crusher becomes tohigh, and the pulverization efficiency becomes low.

The pulverized wastes are subsequently charged into a granulater andformed into a cylindrical solid form. The width of the waste particlesis preferably 2 to 20 mm, more preferably 4 to 6 mm. The length of thewaste particles is preferably 2 to 60 mm. The granulating method is ofthe type of stuffing the pulverized polyester wastes into granulatingholes of a specified size, partially melting the surface portions of thepolyester waste particles by frictional heat generated on the polyesterwaste particle surfaces and solid-forming the pulverized wastes particleutilizing the melted portions as a binder. If size of the pulverizedparticles is too large, the amount of the generated frictional heat isthe small and thus the pulverized particles surfaces are notsufficiently bonded to each other. When the bonding is insufficient asdescribed above, the granulated waste particles are disintegrated bycollision with pipes in the subsequent transportation step, and thuswhen the disintegrated particles are stored in a storage tank, thedisintegrated waste particles form a bridging structure and thus arevery difficult to discharge from the storage tank. The granulationprocedure is performed at a temperature not lower than the glasstransition point of the polyester and not higher than the melting pointof the polyester.

Namely, in another granulating method, the wastes is heated to atemperature not lower than the melting point of the polyester, tocompletely melt the wastes, then the melt is granulated and cooled.However, in this granulating method, impurities such as nylon, in thewastes, are thermally decomposed due to high temperatures to deterioratethe quality of recovered products. Therefore it is preferable to performthe granulating method in which the polyester waste is heated at atemperature equal to or higher than the glass-transition point of thepolyester and equal to or lower than 195° C., so as to pertially meltthe surfaces of the waste particles and the bond the melted surfaces toeach other. In this method, the handleability of the waste particles canbe improved while suppressing the decomposition of the impurities andthe decline of rate of reaction. It is needless to say that, in theabove-mentioned method, there is no trouble at all even if the polyesterwastes are contaminated with resin wastes.

The procedures for preparing a DMT composition and EG, which areprincipal materials for the polyester, from the polyester wastes will beexplained below.

The polyester wastes which have been appropriately pulverized, washedand separated in the above-mentioned pulverization, washing andseparating treatment in the procedures are subjected to a depolymerizingreaction procedure in the presence of a conventional depolymerizationcatalyst in EG. In this procedure, the depolymerizing conditionsdescribed above may be directly adopted for the depolymerizing reaction.

When the present recycling system is adopted, the depolymerizingreaction of the polyester with EG is preferably carried out at atemperature of 110 to 195° C. That is, if the depolymerizing reactiontemperature is below 110° C., the depolymerizing time is extremelyprolonged. On the other hand, if the depolymerizing reaction temperatureis more than 195° C., in the case where the polyester wastes containingnylon are used as a raw material, the nylon is thermally decomposed andthe resultant nitrogen compounds contaminate the DMT and EG and thequality of the resultant polyester products, produced by utilizing theDMT and EG, is unfavorably deteriorated.

The DMT composition obtained by the above-mentioned procedures is thenused together with EG as a raw material for producing the polyester byusing a conventional apparatus for producing a high-purity polymer. AsEG to be used in this case, the EG obtained by the procedures in whichthe polyester wastes is subjected to a depolymerization reaction withEG; the resultant products are subjected to a transesterificationreaction with MeOH; and the resultant crude DMT and EG are separatedfrom each other and refined, is used.

A high-purity polyester polymer of the purity equal to or higher thanthat of the virgin polymer is obtained by the above-mentionedprocedures, and various kinds of polyester products can be produced inthe manner similar to that for the virgin materials by a conventionaltechnique.

Although the method for producing the high-purity polymer is a wellknown method, for reference, the production method of a polyalkyleneterephthalate resin composition will be explained below. A startingmaterials including the DMT composition and EG obtained by theabove-mentioned production equipment are subjected totransesterification reaction procedure in the presence of atransesterification catalyst to prepare bis(β-hydroxyethyl)terephthalateand/or an oligomer thereof, then the transesterification product issubjected to a melt polycondensation procedure in the presence of apolycondensation catalyst and a stabilizer at a high temperature under areduced pressure to provide a polyalkylene terephthalate resincomposition.

The transesterification catalyst preferably comprises one or moremembers of salts of alkaline earth metals, for example, magnesium andcalcium and compounds of metals, for example, titanium, zinc andmanganese. The polycondensation catalyst preferably comprises one ormore members of germanium compounds, antimony compounds, titaniumcompounds, cobalt compounds, tin compounds, etc.

The stabilizer preferably includes phosphate esters, for example, astrimethyl phosphate, triethyl phosphate and triphenyl phosphate;phosphite esters for example, triphenyl phosphite and trisdodecylphosphite, acid phosphate esters, for example, methyl acid phosphate,dibutyl acid phosphate and monobutyl acid phosphate; and phosphoruscompounds, for example phosphoric acid, phosphorous acid,hypophosphorous acid or polyphosphoric acid.

The transesterification catalyst may be fed during the preparation ofthe starting materials and further in the initial stage of thetransesterification reaction. The stabilizer may be fed in or before theinitial stage of polycondensation reaction; however, the stabilizer ispreferably fed when the transesterification reaction is completed.Furthermore, the polycondensation catalyst may be fed in or before theinitial stage of the polycondensation reaction. The reaction temperatureduring the transesterification is usually 200 to 260° C., and thereaction pressure is usually the ambient atmospheric pressure to 0.3MPa. The reaction temperature in the polycondensation is usually 250 to300° C., and the reaction pressure is usually 60 to 0.1 kPa. Thetransesterification and polycondensation reaction as mentioned above maybe carried out in one stage or dividedly in plural stages. The polymerthus prepared has an intrinsic viscosity of usually 0.4 to 0.90 and isformed into chips by a conventional procedure. The average particle sizeof the polymer chips is within the range of usually 2.0 to 5.5 mm,preferably 2.2 to 4.0 mm. The polymer obtained by the meltpolycondensation procedure as described above may be further subjectedto solid-phase polymerization. The polymer chips fed to the solid-phasepolymerization are preheated at a lower temperature than a temperatureat which the solid-phase polymerization is carried out, toprecrystallize and then the precrystallized polymer is fed to thesolid-phase polymerization. In the precrystallization procedure, thepolymer chips in an amorphous state are crystallized in one stage or twostages always under fluidized conditions so as not to cause the polymerchips to be melt-adhered to each other by heat generated bycrystallization of the polymer chips. The next solid-phasepolymerization procedure comprises at least one stage and is carried outat a polymerization temperature equal to or lower than the melt-adheringtemperature of the polymer under conditions of a vacuum of 0.05 to 5 kPaor under a pressure of from the ambient atmospheric pressure to 0.1 MPain a stream of an non-reactive gas, for example, nitrogen, argon orcarbon dioxide. The solid-phase polymerization reaction time may becomeshorter with an increase in reaction temperature, but is usually 1 to 50hours, preferably 5 to 30 hours, more preferably 10 to 25 hours.

The DMT composition of the present invention can be converted intoterephthalic acid by a conventional procedure, for example, hydrolysis,and is used as a starting material for producing the polyester. Thereby,the already existing equipment can be utilized for producing thepolyester polymer using the terephthalic acid as a starting material.The method for producing the high-purity polymer in this case is also awell-known. However, the method will be explained below for reference.

A starting material including the terephthalic acid and EG obtained byhydrolyzing the DMT composition of the present invention is subjected toa esterifying reaction procedure to preparebis(β-hydroxyethyl)terephthalate and/or oligomers thereof, and thereaction product is subjected to a melt polycondensation procedure inthe presence of a polycondensation catalyst and a stabilizer, at a hightemperature under a reduced pressure, to provide the polymer. The use ofthe esterification catalyst is not especially necessary becauseterephthalic acid itself serves as an autocatalyst for theesterification reaction. Germanium compounds, antimony compounds,titanium compounds, cobalt compounds, tin compounds, etc. are generallyknown as polycondensation catalysts, however, the polycondensationcatalyst to be used in the present invention is limited to germaniumdioxide which can impart satisfactory color tone, transparency andhygienic property to the resultant polymer. The amount of the catalystis preferably 20 to 150 ppm (based on the acid components of thepolyester), more preferably 30–100 ppm, still more preferably 30 to 80ppm, expressed in terms of the germanium element.

The stabilizer to be used for the polymerization preferably includesphosphate esters, for example, trimethyl phosphate, triethyl phosphateand triphenyl phosphate; phosphite esters, for example, triphenylphosphite and trisdodecyl phosphite; acid phosphate esters, for examplemethyl acid phosphate, dibutyl acid phosphate and monobutyl acidphosphate; and phosphorus compounds, for example, phosphoric acid,phosphorous acid, hypophosphorous acid or polyphosphoric acid. Theamount of the catalyst used is within the range of usually 5 to 1000ppm, preferably 10 to 500 ppm, based on the total mass of polymerizationmaterials and expressed in terms of the mass of the metal in thecatalyst. The amount of the stabilizer used is within the range ofusually 10 to 1000 ppm, preferably 20 to 500 ppm, based on the totalmass of polymerization materials and expressed in terms of the weight ofthe phosphorus atom in the stabilizer.

The catalyst and stabilizer can be fed in an optional stage of theesterifying reaction and the transesterification in addition to thepreparation of the starting material slurry, and can further be fed inthe initial stage of the polycondensation reaction step. The reactiontemperature in the esterification or transesterification procedure isusually 240 to 280° C., and the reaction pressure is from the ambientatmospheric pressure to 0.3 MPa. The reaction temperature in thepolycondensation procedure is usually 250 to 300° C. The reactionpressure is usually 60 to 0.1 kPa. The esterification reaction ortransesterification and polycondensation reaction may be carried out inone single step or dividedly in plural steps. The polymer thus obtainedhas an intrinsic viscosity of usually 0.45 to 0.70 and is formed intochips by a conventional procedure. The average particle size of thepolymer chips is within the range of usually 2.0 to 5.5 mm, preferably2.2 to 4.0 mm.

The polymer obtained as described above by the melt polycondensationprocedure is usually further fed to the solid-phase polymerization. Thepolymer chips fed to the solid-phase polymerization are preheated at alower temperature than the temperature at which the solid-phasepolymerization is carried out, to precrystallize the polymer chips, andthen they are fed to the solid-phase polymerization. In theprecrystallization step, the polymer chips in an amorphous state arecrystallized at a temperature of usually 120 to 200° C., preferably 130to 180° C. in one stage or two stages for at least 15 minutes or more,while keeping the polymer chips in a fluidized condition to prevent themelt-adhesion of the polymer chips to each other due to the exothermiccrystallization of the polymer chips.

The solid-phase polymerization comprises at least one stage and iscarried out at a polymerization temperature of usually 190 to 230° C.,preferably 195 to 225° C. under a vacuum of 0.05 to 5 kPa or underconditions of atmospheric pressure to 0.1 MPa under the flow of anon-reactive gas such as nitrogen, argon or carbon dioxide. Thesolid-phase polymerization time may become shorter with an increasingtemperature, but is usually 1 to 50 hours, preferably 5 to 30 hours,more preferably 10 to 25 hours. The intrinsic viscosity of the polymerobtained by the solid-phase polymerization is within the range ofusually 0.70 to 0.90.

In the present invention, the content of diethylene glycol (which willbe referred to as DEG hereinafter) in the polyethylene terephthalateobtained by the above-mentioned procedure is 0.7 to 2.0% by mass,preferably 1.0 to 1.5% by mass, based on the total amount of the diolunits constituting the polyethylene terephthalate. If the content of DEGis too low, the transparency of the bottle body section after molding isdeteriorated. If the content of DEG is too high, the heat resistance ofthe product is decreased and, further, accelerating effects oncrystallization are reduced. To adjust the content of DEG within theabove range, the following methods may be employed. A method in whichthe DEG is used as a part of polymerization materials, and a method inwhich a portion of ethyleneglycol used as a principal material isconverted to DEG, and thus the amount of DEG produced as a by-product iscontrolled while the reaction conditions are controlled.

Oligomer components which are contained in the polymer and are a maincause for metal mold staining during the molding of bottles, andacetaldehyde (which will be referred to AA hereinafter) components whichare contained in the polymer and affect taste or smell of materialsfilled in the bottles, are preferably minimized. The oligomer content inthe polymer is preferably 0.5% by mass or less, more preferably 0.4% bymass or less. The content of the AA in the polymer is preferably 5 ppmor less, more preferably 2 ppm or less.

Since the contents of the oligomers and AA are reduced by theabove-mentioned solid-phase polymerization procedure, the target levelsare achieved by controlling the intrinsic viscosity of the polymer afterthe melt polymerization, and the time and temperature of the solid-phasepolymerization.

Furthermore, the concentration of terminal carboxyl groups of thepolymer is especially preferably controlled in the range of from 15 to25 eq/ton. When the concentration of the terminal carboxyl groups fallsbelow the above-mentioned range, the solid-phase polymerizability of theresultant polymer is poor and a long time may be necessary to increasethe intrinsic viscosity of the polymer to the target level. On the otherhand, when the concentration is above the above-mentioned range, andwhen the polymer is subjected to the solid-phase polymerization,reduction effects on the content of cyclic trimeric oligomers may bepoor.

The polyester polymer thus obtained can be formed into polyester filmsby a film-forming equipment to provide a group of various kinds ofpolyester film products, or converted into polyester yarns or fibers bya yarn manufacturing equipment to produce products such as clothes,carpets, interior automotive trims, futons (bedclothes) or flooringmaterials. The above-mentioned polymer can be used as a material for theproduction of PET bottles or engineering plastics by carrying outnecessary treatment of the polymer with solid-phase polymerizationequipment.

When the above-mentioned recycling system is utilized, the group of theoriginal polyester products can be re-produced from the wastescontaining the polyester. Thus, the nearly complete “circulation typerecycling system” can be constructed.

Polyester products, not only the bottles made of the polyester but thefiber wastes, causing great problems at present, can readily be recycledto polyester products, and the necessity for environmental pollutiontype disposal such as landfilling or incineration as general industrialwastes is eliminated. Therefore, the recycling system is effective inenabling the solution to waste disposal problems and achievement ofresource saving and energy saving.

The transesterifying equipment, crude DMT refining equipment, hydrolyticequipment, monomer producing equipment, polymer producing equipment,EG/MeOH refining equipment, etc. used in the system are alreadyestablished as individual equipment. Thus, according to the presentrecycling system, when an improvement is applied to the pretreatingequipment and depolymerization equipment, and the improved equipmentsare combined with the above-mentioned conventional equipments, acirculation type recycling system intended for polyester wastes such asused bottles, clothes, films, etc. made of polyester, can beconstructed, to turn the polyester wastes into original products.

EXAMPLES

The contents of the present invention will be further explained, inspecific detail, by the following examples which are not intended tolimit the scope of the present invention in any way. Respective data inthe examples are determined by the following methods:

(1) Qualitative Analysis of 4-DOMB and HDT:

A sample was subjected to recrystallizing and extracting proceduresusing acetone solvent and MeOH solvent, and the resulting extract wasthen concentrated. The concentrated sample was subjected to aqualitative analysis using gas chromatography (apparatus: HP5890manufactured by Hewlett Packard Co.; capillary column: DB-17manufactured by J&W Scientific, Inc.) in a guaranteed reagent acetonesolvent.

(2) Quantitative Analysis of 4-DOMB and HDT:

A sample was subjected to recrystallization and extraction operationsusing acetone solvent and MeOH solvent, and the resulting extract wasthen concentrated. The concentrated sample was subjected to aquantitative analysis using GC-MASS (apparatus: GC/massdetector=HP6890/HP5973 manufactured by Hewlett Packard Co.; capillarycolumn, DB-17 manufactured by J&W Scientific, Inc.) in a guaranteedreagent acetone solvent.

(3) Fluorescence Intensity of DMT:

A sample was subjected to a fluorescence intensity measurement at anexcitation wavelength of 328 nm and a fluorescence wavelength of 454 nmin chloroform used as a measuring solvent. F-4500 manufactured byHitachi, Ltd. was used as the fluorophotometer.

(4) Compositional Ratios of bis-β-hydroxyethylene terephthalate (whichwill be Referred to BHET hereinafter) and Lower Oligomer Components:

The compositional ratios were determined by using GPC (apparatus: L-4000liquid chromatograph manufactured by Hitachi, Ltd., tetrahydrofuransolvent), by a conventional method.

(5) Alkali Transmittance of Terephthalic Acid:

A solution was obtained by using 7.5 g of terephthalic acid in 50 ml (2mol/L) of an aqueous solution of potassium hydroxide, and the alkalitransmittance was determined from transmittance at a wavelength of 340nm with an optical path length of 1 cm according to the method describedin “CHEMICAL ENGINEERING OF JAPA”N, vol. 58, No. 10, pp. 787–789(published by THE SOCIETY OF CHEMICAL ENGINEERS, JAPAN, 1994).

(6) Intrinsic Viscosity:

A predetermined amount of a sample cut out from a chip or a moldedproduct was weighted, dissolved in o-chlorophenol at a concentration of0.012 g/ml and the resultant solution was subjected to a measurement ofthe intrinsic viscosity, at 25° C.

(7) Haze:

A measurement of haze of a sample cut out from a bottle body section toa size of 50 mm×50 mm was carried out with a color and color differencemeter (MODEL1001DP) manufactured b Nippon Densyoku K.K. Content of AA:

(8) Content of AA

The content of AA was determined by freeze-pulverizing a sample,charging the pulverized sample into a vial, keeping the sample at 150°C. for 60 minutes and the content of AA in the sample was measured witha headspace-gas chromatograph manufactured by Hitachi, Ltd.

(9) Content of DEG:

A sample was decomposed with hydrazine, and measurement of DEG contentof the decomposed sample was made by gas chromatography.

(10) Col-b:

A predetermined volume of a sample was taken, and measurement of b*value of the sample was made with a color machine CM-7500 modelmanufactured by Color Machine Co.

Reference Example 1

DMT manufactured in the form of a white briquette by Petrocel Temex S.A.was subjected to a microanalysis. As micro content components, p-toluicacid and monomethyl terephthalate as acid components; dimethylisophthalate and dimethyl phthalate as isomers; and methylgroup-substitution products, for example, methyl p-toluate,4-carbomethoxybenzaldehyde and dimethyl (o-, m-, p-)phthalate as otheresters were detected. However, 4-DOMB was not detected.

In the DMT briquette, 1.5 ppm of HDT was detected, and the fluorescenceintensity of the DMT briquette was further measured, and the resultantintensity value was 900.

Reference Example 2

A high-purity terephthalic acid (which will be referred to as PTAhereinafter) manufactured by Mitsui Chemicals, Inc. was subjected to ameasurement of alkali transmittance.

The measurement result was 91%.

Example 1

A 500-ml separable flask was charged with 200 parts of EG, and furthercharged with 1.5 parts of sodium carbonate and 50 parts of polyethyleneterephthalate. The temperature of the resultant mixture was increasedwhile stirring the mixture at a stirring speed of 100 rpm to provide aninternal temperature of the flask of 185° C. The mixture was kept in theabove-mentioned conditions for 4 hours to complete a depolymerizingreaction of the polymer. The resultant depolymerized product was thenconcentrated by a distillation procedure under a reduced pressure of6.65 kPa to recover the resultant concentrate and 150 parts of EG as adistilled fraction.

The resultant concentrate was mixed with 0.5 part of sodium carbonate asa transesterification catalyst and 100 parts of MeOH, and the mixturewas kept at a liquid temperature of 75° C., under the ambientatmospheric pressure at a stirring speed of 100 rpm for 1 hour to effecta transesterification reaction.

The obtained mixture was cooled to 40° C. and filtered through a 3G-4filter made of glass. The crude DMT recovered on the filter was mixedinto 100 parts of MeOH, heated to 40° C., while stirred to wash thecrude DMT, and filtered again through a filter made of glass. Thewashing procedure was repeated twice.

The crude DMT collected on the filter was charged into a distillationapparatus and subjected to a distillation procedure under a reducedpressure of 6.65 kPa and a reflux ratio of 0.5. As a result, a DMTcomposition was obtained as a distilled fraction. The recovereddistilled fraction was in an amount of 40 parts. The amount of DMTcontained in the residue in the distillation column was 2 parts. Thereaction yield of the DMT was 93% by mass based on the amount of thepolyethylene terephthalate fed to the flask.

In the DMT composition refined by the distillation, 20 ppm of 4-DOMB and0.5 ppm of HDT were detected. When the 4-DOMB and HDT in the recoveredDMT are compared with standard 4-DOMB and HDT, respectively, by GC-MASSanalysis, it was confirmed that the detected fragment ions wereidentical to each other and thus both of the compared compounds had thesame structure as each other, respectively.

The resultant DMT composition had a degree of purity of 99.9% by mass ormore, an acid value of 0.003 mg (KOH)/g (DMT) and a fluorescenceintensity of 330. Other properties of the resultant DMT were equal tothose of the standard DMT manufactured by Petrocel Temex S.A. inReference Example 1.

Example 2

Steam is continuously blown into 40 parts of the DMT compositionobtained in Example 1 while keeping the conditions of the DMTcomposition at a temperature of 250° C. under a pressure of 3.92 MPa,and excessive portion of the steam and produced MeOH were continuouslywithdrawn to promote a hydrolytic reaction of the DMT composition. Thereaction almost quantitatively proceeded to produce 33 parts ofterephthalic acid.

Into 30 parts of the resultant terephthalic acid, were mixed 60 parts ofMeOH. After the terephthalic acid was washed with MeOH, while stirringat 40° C., the washed terephthalic acid was collected by filtration anddried. The alkali transmittance of the obtained terephthalic acid wasmeasured. As a result, the alkali transmittance was 90%, and nomeaningful difference in the alkali transmittance between the resultantterephthalic acid and the PTA manufactured by Mitsui Chemicals, Inc. wasfound.

Example 3

A separable flask was charged with 40 parts of the DMT compositionobtained in Example 1 and 75 parts of EG, and the resultant mixture washeated with stirring at 100 rpm. When the temperature reached about 200°C., MeOH was generated. The initiation of reaction was confirmed. Thedistilled MeOH fraction was delivered to the outside of the systemthrough two separation columns, and EG and DMT fractions distilledtogether with MeOH were separated from MeOH and returned into the flask.

The above mentioned operations were repeated, and the reaction wasterminated when the inside temperature of the flask reached atemperature of 220 to 250° C. The necessary reaction time was about 8hours.

The composition of the resultant mixture was analyzed by gaschromatography, and it was found that the DMT completely disappeared inthe reaction. The BHET existing in an amount of 45% by mass wasconfirmed by the above-mentioned GC analysis and a GPC analysis. Othercomponents respectively exhibited a sharp molecular weight distribution,and it could be confirmed that the other components were lower oligomersof dimers to pentamers.

Example 4

The DMT composition obtained in Example 1 was used as a startingmaterial to produce PET according to a conventional method. Manganeseacetate catalyst was used, and a transesterification (EI) reaction wascarried out under the ambient atmospheric pressure until the reactiontemperature reached 245° C. to produce lower oligomers comprising, as aprincipal component, BHET. Antimony trioxide was subsequently added tothe resulting lower oligomers comprising, as a principal component BHET,and the mixture was subjected to polymerization at 290° C. under a highvacuum of 0.1 kPa for 1.8 hours. The intrinsic viscosity of theresultant polymer was 0.70. The analytical results in anothercharacteristics such as color, thermal characteristics and the contentof DEG were almost the same as those of a polymer produced from the DMTmanufactured by Petrocel Temex S.A. shown in Reference Example 1.

A gas chromatographic analyser equipped with a headspace sampler device(HS40XL manufactured by Perkin Elmer Corp.) was used to detect volatilecomponents of the obtained polymer under conditions of 200° C. for 60minutes. As a result, no 4-DOMB or HDT was detected.

Comparative Example 1

The same procedures as in Example 1 were carried out except that thevacuum distillation of crude DMT was carried out under distillationconditions of a reduced pressure of 6.65 kPa and a reflux ratio of 0.05,to provide a DMT composition as a distilled fraction. The disitilledfraction was recovered in an amount of 40 parts, amount of the residuein the bottom of the distillater were measured, and the amount of theDMT was determined. As a result, the amount of DMT was 2 parts. Thereaction yield of the DMT based on the amount of the charged polyesterwas 93% by mass.

In the DMT composition refined by the distillation, 40 ppm of 4-DOMB and1.1 ppm of HDT were detected.

With respect to the quality of the refined DMT composition, the DMTcomposition had a degree at purity of 99.9% by mass or more and an acidvalue of 0.003 mg (KOH)/g (DMT), and a exhibited a high fluorescenceintensity of 700.

Comparative Example 2

The same procedures as in Example 1 were carried out except that theresulting mixture obtained by the transesterification reaction wascooled to 40° C. and filtered through a 3G-4 filter made of glass. Thecrude DMT collected on the filter was mixed in an amount of 45 partsinto 40 parts of MeOH. The resultant mixture was heated to 40° C.,stirred to wash the crude DMT with MeOH and refiltered through a filtermade of glass. The washing was repeated twice.

A distillation apparatus was charged with the crude DMT which wascollected on the filter, the charged DMT was distilled under a reducedpressure of 6.65 kPa and a reflux ratio of 0.5, to collect DMT as adistilled fraction. The distilled fraction was recovered in an amount of40 parts. The amount of the residue in the bottom of the distillationapparatus was measured, and the amount of the DMT in the residue wasdetermined. The amount of DMT was 2 parts. The reaction yield of the DMTbased on the amount of the charged polyester was 93% by mass.

In the DMT composition refined by the distillation, 250 ppm of 4-DOMBand 0.5 ppm of HDT were detected.

With respect to the quality of the refined DMT composition, the degreeof purity was 99.8% by mass, and the acid value was 0.01 mg (KOH)/g(DMT). A DMT composition which had the quality equal to that of the DMTmanufactured by Petrocel Temex S.A., shown in Reference Example 1, couldnot be obtained.

Example 5

Bales (120 kg bales having bale dimensions: 900 mm×1000 mm×550 mm) ofpolyester bottles which were classified, collected and recovered bymunicipalities were opened, and then the polyester bottles was fed intoa front crusher, having a screen opening diameter of the crusherestablished at 75 mm and pulverized by the front crusher. The resultingcrushed material was then charged into a second crusher in which thescreen opening diameter of the crusher was established at 10 mm, andpulverized in the second crusher.

The resultant pulverized material was subsequently treated with an airblow separator to remove labels, attached to the polyester bottles, andcomprising, as principal components, polyethylene, polystyrene and/orpolypropylene. Then, centrifugal separation with a decanter was appliedto the separated fraction to remove caps of the bottles comprising as aprincipal component, polypropylene or polyethylene and labels which werenot removed by the air blow separation, while washing off the contentsof the bottles with water and removing the contents. Thereby, recoveredflakes were obtained. The resultant recovered flakes in an amount of 100kg were transported to a reaction step by pneumatic transportation.

In the reaction step, the recovered flakes were charged into a mixtureconsisting of 400 kg of EG with 3 kg of sodium carbonate and preheatedto 185° C. and the mixture was subjected to a reaction under the ambientatmospheric pressure for 4 hours. Plastics other than the polyester, forexample, polystyrene, which could not be removed in the pretreating stepand floated to the EG liquid surface, were removed herein bysolid-liquid separation. After completing the reaction, EG was distilledoff in an amount of 300 kg under conditions of 140 to 150° C. and apressure of 13.3 kPa. After distilling off the EG, the resultant residuein an amount of 200 kg was mixed with 3 kg of sodium carbonate and 200kg of MeOH. The mixture was subjected to a reaction at 75 to 80° C.under the ambient atmospheric pressure for 1 hour.

After completing the reaction, the reaction liquid was cooled to 40° C.,subjected to solid-liquid separation using a centrifugal separation, andseparated into a cake comprising, as a principal component, a crude DMTand a filtrate comprising as principal components, MeOH and crude EG.The crude DMT recovered by the solid-liquid separation was mixed into200 kg of MeOH, the mixture was heated to 40° C. and stirred, to washthe crude DMT, and the mixture was further subjected to a solid-liquidseparation by centrifugal separation. The washing procedure was repeatedtwice.

The resulting crude DMT was then refined by distillation underconditions of a pressure of 6.7 kPa and a reflux ratio of 0.5, and thecrude EG was refined by distillation under conditions of a pressure of13.3 kPa and a column bottom temperature of 140 to 150° C. As results, aDMT composition and EG were finally obtained in a yield of about 85%,respectively. The recovered DMT composition was by no means inferior tocommercially available products in inspection items of appearance, acidvalue, melting colorimetry and sulfuric acid ash content. In the DMTcomposition, 22 ppm of 4-DOMB and 0.6 ppm of HDT were detected.

The resultant DMT composition was hydrolysed, and the resultantterephthalic acid (which will be referred to as recycled TA hereinafter)in an amount of 40 kg was mixed with 22 kg of ethylene glycol to providea slurry. The slurry was fed into a polycondensation vessel andsubjected to an esterifying reaction at 275° C. under the ambientatmospheric pressure for 4 hours. The reaction was carried out until theesterifying conversion rate reached 97%, while water generated as aby-product was discharged to the outside of the system, to prepareoligomers having a degree of polymerization of 5 to 10. To the preparedoligomers, 0.017 kg of an EG solution of phosphoric acid (at aconcentration of 5.5% by weight in terms of phosphorus element) and 0.38kg of an ethylene glycol solution of germanium dioxide (at aconcentration of 1% by weight expressed in terms of germanium dioxide)were mixed. The mixture was subjected to a polycondensation under areduced pressure of 2 kPa for 1 hour and then at 277° C. under a reducedpressure of 133 Pa for 2 hours. The produced polymer was withdrawn inthe form of a strand, from a takeout port which was provided at thebottom of the polycondensation vessel and directly connected to acooling water tank, the withdrawn product was cooled with water and thencut into the form of chips to prepare polymer chips.

The resulting polymer chips were subsequently crystallized in astir-fluidizing type crystallizer, then dried at 140° C. under the flowof a nitrogen gas for 3 hours, subsequently transferred to a packedcolumn type solid-phase polymerization column and subjected to thesolid-phase polymerization at 215° C. under the flow of nitrogen for 22hours, to produce a polyethylene terephthalate resin composition in theform of chips.

The resultant polyethylene terephthalate resin composition chips had anintrinsic viscosity of 0.75, a content of DEG of 1.3% by mass, a contentof AA of 1.5 ppm and a Col-b^((*)) value of −1.0.

The chips were dried at 160° C. for 5 hours by using a dryer and theninjection-molded by using an injection molding machine (“M-100DM”manufactured by Meiki Co., Ltd.) at a cylinder temperature of 275° C., anumber of revolutions of screw of 160 rpm, a primary pressing time of3.0 seconds, a mold temperature of 10° C. and a cycle time of 30 secondsto produce a cylindrical preform having an outside diameter of about 28mm, an inside diameter of about 19 mm, a length of 136 mm and a mass ofabout 56 g. The resulting preform had an intrinsic viscosity of 0.69 anda content of AA of 12 ppm, and the moldability and appearance of thepreform were good.

The surface of the preform was subsequently preheated to a temperatureof about 110° C. by using an infrared ray heater, subjected to stretchblow molding procedure using a blow molding machine under a blowingpressure of 5 to 40 kg/cm² at a metal mold temperature of 150° C., toproduce bottles having an average wall thickness in the body section of330 μm and an internal volume of about 1.5 liters. The resultant bottleshad a haze of 0.8% and exhibited good moldability and appearance.

Example 6

An absorption spectrum of 100 kg of a mixed waste of polyester fiberwastes which were generated from a polyester production process andcontained no dye, with polyester film wastes, was measured by anear-infrared ray analyzer and it was found the measured absorptionpattern was identical to that of polyester.

The whole amount of the mixed waste was then charged into a frontcrusher, and primarily crushed. In this front crusher the screen openingsize of the front crusher was set at 75 mm. The resultant crushed wastewas then charged into a second crusher, and secondarily crushed. In thissecond crusher, the screen opening size of the crusher was set at 20 mm.The resultant crushed waste was subsequently charged into a granulateroperated at an internal temperature of 170° C., to produce granuleshaving a diameter of 4 mm and a length of 45 mm, and then the resultantgranules were transported to a reaction step through a pneumatictransportation means. The bulk density of the granulated wastestransported to the reaction step was 0.40 g/cm³.

In the reaction step, 100 kg of the resulting granulated waste was mixedinto a mixture of 400 kg of EG and 3 kg of sodium carbonate which werepreheated to 185° C. The mixture was subjected to a reaction under theambient atmospheric pressure at the above-mentioned temperature, for 4hours.

After completing the reaction, 300 kg of EG was distilled off from thereaction product under conditions of 140 to 150° C. and a pressure of13.3 kPa, and 3 kg of sodium carbonate and 200 kg of MeOH were added to200 kg of the residue after the EG was distilled off the EG, and themixture was subjected to a reaction at 75 to 80° C. for 1 hour.

After the reaction was completed, the resultant reaction liquid wascooled to 40° C., subjected to solid-liquid separation by which thereaction liquid was separated into a cake comprising, as principalcomponent, crude DMT, and a filtrate comprising, as principalcomponents, MeOH and a crude EG, by centrifugal separation. The crudeDMT recovered by the solid-liquid separation was mixed into 200 kg ofMeOH, the mixture was heated at 40° C. and stirred, to wash the crudeDMT and then subjected again to solid-liquid separation by centrifugalseparation. The washing operation was repeated twice.

The crude DMT was refined by distillation under conditions of a pressureof 6.7 kPa and a reflux ratio of 0.5 and the crude EG was refined bydistillation under conditions of a pressure of 13.3 kPa and a columnbottom temperature of 140 to 150° C. As a result, a DMT composition andEG were finally obtained in a yield of 85%, respectively. The recoveredDMT composition was by no means inferior to commercially available DMTcomposition in inspection items of appearance, acid value, meltcolorimetry and sulfuric acid ash content, and in the DMT composition,25 ppm of 4-DOMB and 0.6 ppm of HDT were detected.

The recovered EG was by no means inferior to commercially available EGin inspection items of content of DEG, moisture content and meltcolorimetry.

Then, 50 kg of the resulting DMT composition and 32 kg of EG weresubjected to a transesterification reaction using a transesterificationcatalyst comprising tetra-t-butoxytitanium while MeOH generated as aby-product was distilled away to the outside of the reaction system.Then, the reaction mixture was further mixed with germanium dioxide as apolymerization catalyst and the resultant mixture was subjected to atransesterification reaction, while the mixture was heated to atemperature to 250° C. At a stage in which the distilling off of theMeOH was almost completed, orthophosphoric acid as a stabilizer wasadded to complete the transesterification reaction.

The reaction product was subjected to polycondensation reaction at ahigh temperature under a high vacuum. A polymer having an intrinsicviscosity of 0.60 was obtained. The resultant polymer was subsequentlysubjected to solid-phase polymerization procedure to produce apolyalkylene terephthalate resin composition having an intrinsicviscosity of 0.83, a Col-b* value of 2, a content of DEG of 1.8% by massand a content of AA of 2 ppm.

The resultant polyalkylene terephthalate resin composition was dried at160° C. for 5 hours by using a dryer and then, in a first workingexample, the dried resin composition was subjected to an injectionmolding procedure using an injection molding machine (“M-100DM”manufactured by Meiki Co., Ltd.) at a cylinder temperature of 275° C., anumber of revolutions of screw of 160 rpm, a primary pressing time of3.0 seconds, a metal mold temperature of 10° C. and a cycle time of 30seconds to form cylindrical preforms having an outside diameter of about28 mm, an inside diameter of about 19 mm, a length of 136 mm and a massof about 56 g.

The intrinsic viscosity of the resultant preforms was 0.77 and theappearance and injection moldability were good. The surfaces of thepreforms were subsequently preheated to a temperature of about 110° C.with an infrared ray heater, and the preheated preforms were subjectedto a blow draw-molding procedure using a blow molding machine under ablowing pressure of 5 to 40 kg/cm² and a mold temperature of 150° C. andmolded into bottles having an average wall thickness in the body sectionof about 330 μm and an internal volume of about 1.5 liters. The haze ofthe bottles was 0.9%, and the drawability was good.

In a second working example, a sheet having a film thickness of 0.5 mmwas produced from the dried polyalkylene terephthalate resin compositionby using a vented bi-axial extruder, subsequently heated at a sheetsurface temperature of 100° C. and thermoformed into a tray withcompressed air. The haze of the resultant tray was 1.0%, and moldabilitythereof was good.

Reference Example 3

The same procedures as in Example 5 were carried out except that theterephthalic acid was replaced by PTA manufactured by Mitsui Chemicals,Inc. The resultant polyethylene terephthalate resin composition had anintrinsic viscosity of 0.75, a Col-b* value of −1.5, a content of DEG of1.3% by mass and a content of AA of 1.3 ppm.

The resin composition was subjected to the same injection moldingprocedure as in Example 5, to produce performs, and the resultantpreforms had an intrinsic viscosity of 0.69 and a content of AA of 12ppm, and moldability and appearance were good. Also, the resincomposition was subjected to the same blow molding procedure as inExample 1, to provide bottles. The resultant bottles had a haze of 1.0%and moldability and appearance of the bottles were good.

Reference Example 4

The same procedures as in Example 6 were carried out, except that DMTmanufactured by Teijin Ltd. was used in place of the DMT used in Example6. A polyalkylene terephthalate resin composition having an intrinsicviscosity of 0.82, a Col-b of 2, a content of DEG of 1.8% by weight anda content of AA of 2 ppm was obtained.

The resultant polyalkylene terephthalate resin composition was moldedinto preforms in the same manner as in Example 6. The resultant preformshad an intrinsic viscosity of 0.76, and appearance and injectionmoldability of the preforms were good. Also, the resin composition wassubjected to a blow molding procedure in the same manner as in Example6, to form bottles. The resultant bottles had a haze of 0.9%, and thedrawability of the bottles was good.

Furthermore, a tray obtained from the resin composition by thermoformingin the same manner as in Example 6 had a haze of 1.1%, and moldabilitythereof was good.

Comparative Example 3

The same procedures as in Example 5 were carried out, except thatpolyester wastes prepared by cutting a polyester and nylon blended yarnuniform wastes into a size of 10 to 20 mm square were used as fiberwastes; the fiber wastes in an amount of 100 kg was mixed with 65 kg ofEG; and the mixture was subjected to a depolymerization reactionprocedure under conditions of 205° C. and an internal pressure of 0.25MPa.

The quality of the resulting DMT composition was significantly worsethan that of commercially available products in all the inspection itemsof appearance, acid value, melt colorimetry and sulfuric acid ashcontent. The recovered DMT and EG were contaminated with nitrogencompounds and could not be reused as materials for polyester products.In the resultant DMT composition, 30 ppm of 4-DOMB and 1.1 ppm of HDTwere detected.

INDUSTRIAL APPLICABILITY

A dimethyl terephthalate composition, improved in characteristics as astarting material for a polyester, can be provided according to thepresent invention.

When a by-product from a process for recovering DMT is utilized as thedimethyl terephthalate composition of the present invention, a recyclingsystem for polyester wastes can be constructed. This system is acirculation type recycling system capable of producing dimethylterephthalate and ethylene glycol each having a high degree of purityfrom usual polyester wastes containing the polyester (for example PETbottles, uniforms, futons (bedclothes), films, etc.), reproducing agroup of polyester products using the dimethyl terephthalate andethylene glycol as starting materials and recycling the used polyesterproducts into polyester products having a quality equal to that of theoriginal products. Thereby, the necessity for performing landfilling orincinerating treatment of general industrial wastes is eliminated, andresource saving or energy saving can be achieved.

1. A dimethyl terephthalate composition comprising, as a principalcomposition, dimethyl terephthalate, and further containing 0.001 to 200ppm of methyl 4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate.
 2. A method for producing a dimethyl terephthalatecomposition comprising, as a principal component, dimethylterephthalate, and further containing 0.001 to 200 ppm of methyl4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate, comprising: subjecting a polyalkyleneterephthalate to a depolymerization reaction with ethylene glycol;subsequently subjecting the resulting mixture to a transesterificationreaction with methanol, to produce crude dimethyl terephthalate and analkylene glycol; applying a cooling treatment to the reaction mixture;carrying out operations to separate the reaction mixture into a cake ofcrude dimethyl terephthalate and a mixture solution by using acentrifugal separator; and then distillation-refining the cake toprovide a dimethyl terephthalate composition, characterized in that thereflux ratio during the distillation-refining step is controlled to 0.1to
 2. 3. The method for producing the dimethyl terephthalate compositionaccording to claim 2, wherein the polyalkylene terephthalate comprisesat least one member selected from the group consisting of polyethyleneterephthalate, polytrimethylene terephthalate and polybutyleneterephthahate.
 4. A method for producing a dimethyl terephthalatecomposition comprising, as a principal component, dimethyl terephthalateand further containing 0.001 to 200 ppm of methyl4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate, comprising: subjecting a polyalkyleneterephthalate to a depolymerization reaction with ethylene glycol;subsequently subjecting the resulting mixture to a transesterificationreaction with methanol, to produce crude dimethyl terephthalate and analkylene glycol; applying a cooling treatment to the reaction mixture;separating the reaction mixture into a cake of crude dimethylterephthalate and a mixture solution by using a centrifugal separator;and then distillation-refining the cake to provide a dimethylterephthalate composition, characterized in that methanol is added in anamount of 1 to 5 times the mass of the cake obtained by centrifugalseparating operation to the cake under conditions of 30 to 60° C.; thenthe resulting mixture is again subjected to a separation operations toseparate a cake of dimethyl terephthalate from a mixture solution byusing a centrifugal separator; and then the resultant cake is subjectedto a distillation refining procedure.
 5. The method for producing thedimethyl terephthalate composition according to claim 4, wherein thepolyalkylene terephthalate comprises at least one selected from thegroup consisting of polyethylene terephthalate, polytrimethyleneterephthalate and polybutylene terephthalate.
 6. A method for producingterephthalic acid, employing as a starting material, a dimethylterephthalate composition consisting, as a principal component, dimethylterephthalate and further containing 0.001 to 200 ppm of methyl4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate.
 7. A method for producingbis(β-hydroxyethyl)terephthalate, employing as a starting material, adimethyl terephthalate composition comprising, as a principal component,dimethyl terephthalate and further containing 0.001 to 200 ppm of methyl4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate.
 8. A method for producing a polyalkyleneterephthalate, employing, as a starting material, a dimethylterephthalate composition consisting, as a principal component, dimethylterephthalate and further containing 0.001 to 200 ppm of methyl4-(1,3-dioxolan-2-yl)benzoate and 0 to 1 ppm of dimethylhydroxyterephthalate.